It is generally not possible to directly measure the flux and torque in a rotating electrical machine, which values may be needed for controlling the rotating electrical machine.
For example, in long motor feeds for oil platforms, the motor (being the electrical machine) and the inverter are separated by long distances, which may be up to ten or more kilometers. For such applications, the control approach may be based on the pulse width and an open loop scalar mode where the resistive voltage drops cannot be estimated accurately. The open loop control is integrated with dynamic torque control (DTC) and vector control approaches, and the same hardware measurements and diagnostic may be applied.
The errors in the resistive voltage drop may influence the accuracy of the estimated flux and the estimated torque. There may be other error sources which may cause an erroneous estimated flux, such as measuring gain and offset errors in the DC intermediate voltage and phase currents, inaccurate estimates of inverter power switches losses or the used integration algorithm.
One known solution for compensating the errors may be to estimate the resistive losses on line and use the estimation in the integration loop. Another possible solution may be to add the measurement of the motor voltage and to apply this to the flux calculation.
A further method may be based on the estimates of the minimum and the maximum flux level within one period and add the average obtained that way to the flux estimate or to directly apply the correction to the voltage vector. However, this may have the disadvantage that it may not result in a good correction, when the frequency of the motor is changing.
Solutions for compensation flux errors may be found in US 2002 0041171 A1 relating to flux observation, TW 441161 A relating to on line parameters adaptation, and JP 2006 136053 A showing a flux offset correction control unit which corrects the electric current using the calculated offset components.